单选题 The pathway of CO2 assimilation has a greater energy cost in C4 plants than in C3 plants. For each molecule of CO2 assimilated in the C4 pathway, a molecule of phosphoenolpyruvate (PEP) must be regenerated at the expense of two phosphoanhydride bonds in ATP. Thus C4 plants need ___ ATP molecules to assimilate one molecule of CO2, whereas C3 plants need only three (nine per triose phosphate)